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SHARAD is a frequency-modulated (15-25 MHz) radar sounder that probes the upper few kilometers of the Martian crust and polar layered deposits. At solar zenith angles less than about 100°, the ionosphere of Mars can induce phase distortion in surface and subsurface radar echoes that substantially degrades the signal-to-noise ratio and vertical resolution of the range-compressed data. We present a range-compression autofocus approach that estimates the phase distortion of SHARAD data along ground-track segments of about 100 km, using a power-law image-sharpness metric and an empirically derived scaling between the phase correction and radar frequency. This method is rapid, yields a greatly improved subsurface image, and provides a means to track regional and temporal changes in the Martian ionosphere.